The present invention relates to a fixing device for an electrophotographic copier, laser printer, facsimile transceiver or similar image forming equipment and, more particularly, to a fixing device capable of controlling the amount of silicone oil to be applied to a heat roller thereof.
A fixing device incorporated in image forming equipment such as an electrophotographic copier has a heat roller having a heater thereinside, a press roller contacting the heat roller, and a piece of felt for applying silicone oil to the heat roller. A prerequisite with this type of fixing device is that silicone oil be applied in as small an amount as possible and in a uniform distribution to the surface of the heat roller. To meet this requirement, a piece of felt or similar stationary member may be held in contact with the surface of the heat roller, as customary in the art. This traditional scheme, however, increases the torque for driving the heat roller and is apt to damage the stationary member and roller to reduce their service life. In the light of this, a system of the type using a plurality of rollers for applying silicone oil to the heat roller is predominant today. This predominant type of system has a first application roller contacting the heat roller for applying silicone oil thereto, a second application roller contacting the first roller, and an oil reservoir storing silicone oil. A piece of felt is immersed in the silicone oil in the reservoir at one end thereof and held in contact with the surface of the second application roller at the other end thereof. A cleaning and oil collecting roller cleans the heat roller to collect the silicone oil. A scraper blade scrapes offset toner particles, paper dust and other impurities off the surface of the cleaning and oil collecting roller.
The conventional fixing device described above lacks means for controlling the amount of silicone oil to be applied to the second application roller, i.e., for regulating the silicone oil fed from the felt to the second application roller to a predetermined small amount. Specifically, the silicone oil fed to the application roller is directly transferred to the other application roller and then to the heat roller. Assume that paper sheets of relatively small size such as format A4 are successively passed through between, for example, the heat roller and press roller. Then, a necessary amount of silicone oil is successfully applied and consumed in a surface portion of the heat roller which the paper sheet passes. However, in the other surface portion of the heat roller 1 where the paper sheet does not exist, the silicone oil is applied to the heat roller in the maximum amount with which the felt can be impregnated. When a paper sheet of relatively large size such as format A3 arrives at the first fixing section while an excessive amount of silicone oil exists in the particular surface portion of the heat roller as mentioned above, a strip-like area of the paper sheet of format A3 corresponding to the above-mentioned surface portion of the heat roller is contaminated.